Hi All,

Here's an odd request
I want to place various electronic devices in my microwave oven (whilst on!) but I do not want to do this without looking into adequate screening for the devices first. The info I can find on the web is limited and contradictory so if anyone has done this before or knows a reliable method of shielding then I would love to know.

The devices will be powered whilst inside the (powered) microwave and the shielding needs to be either transparent (unlikely unless Indium Tin Oxide (or similar) is conductive enough) or like a grid (Faraday cage style) such that a camera can focus through it.

Any ideas appreciated

S.

Well, the grid of the door itself prevents leakage at that frequency, so perhaps you could take some old microwaves apart and use the grids out of their doors to make a faraday cage from, with solid material everywhere else the camera doesn't have to see.

Designing a screened enclosure for use in close proximity to intense SHF radiation requires very careful consideration.

As suggested, the mesh over a microwave door does indeed screen the user against excessive uW radiation, but the mesh is flat and the user is not constantly in adjacent proximity to the door (square law of field strength and distance being what it is), all of which are factors considered in a "safe" microwave cooker design. The downside of a camera inside a screened enclosure, sitting inside a microwave oven, is a considerably more challenging environment - particularly for sensitive electronics which at-least should not be adversely affected if the intended application is to have merit at-all.

To begin, the screened enclosure should offer minimal electrical surface resistance, suggesting a coating of silver, nickel, or similar material with low resistivity due to the skin effect at high-frequency. A material of significantly higher electrical resistance can increase the propensity for re-radiant "hot-spots" to develop - particularly around screws or fixings.

The box should have rounded corners to minimise the propensity for fundamental or harmonic resonance created by travelling waves across its surfaces (this would suggest a sphere as the most effective form of screened enclosure).

The thickness of the material has a bearing on screening effectiveness, as although travelling waves at high frequencies are constrained to the material surface, there is nevertheless an EM coupling to re-radiant inner surfaces of the box. Notably, Mu-metal is finding modern application for screened enclosures, as it has the effect of screening (effectively "repelling") magnetic fields - even static magnetic fields. Externally coat an enclosure made of MuMetal, with a sprayed metal film offering low surface resistance, and you should have a pretty good screened enclosure on your hands

An aperture offering electrical screening whilst minimising interference to light can often be supplemented by ITO (Indinium Tin Oxide) coated glass, though sadly the effects are best observed in the tens of MH'z region. It's effects are reflective (rather than conductive), offering typical figures of around 50-to-60dB attenuation at HF to only around 10-20dB at 1GHz, so hardly a worthwhile proposition for this application.

For the geeks, here's a pretty good online start for screened enclosure calculations (rarely accurate but a good relative indicator as a starting point)...

http://www.emc.york.ac.uk/examples/screening/screening.html

If it were me, I would try experimenting with a small, inexpensive camera - removing the conventional lens and placing the bare CCD (with battery and and accompanying circuitry) inside a small screened ball or "curvy" enclosure - maximising screening efficiency by employing "pin-hole" focus on an externally-chamfered hole. You'd need plenty of light inside the uWave oven, but no need to mess around with camera focus... The optimum subject distance (capture area) would simply be the same as the size of the CCD surface and its distance from the pin-hole inside the screened enclosure, but will always remain in-focus with only a tiny pin-hole to optimise screening efficiency

Just a whacky early-morning thought


All the best,

FJ

I guess I never really thought about the fact there is still leakage, plus surface conduction problems, etc. Never needed to try this sort of thing.

Another alternate possibility would be to use fiber optics to conduct the light from inside the microwave to the camera on the outside. Designing and building the physical interface at each end of the FO might be challenging, if not using off-the-shelf parts.

I once used a bundle of FO out of a bunch of broken glass-fiber FO token-ring network cables, all in parallel, ends cut as near to flat as I could get, to conduct enough light from one room into another that if you squinted and looked at just the right angle into it, you could see some things in the other room. Never tried it with a camera, but it should work similarly.

Won't help any other electronics you have to put in there, though.

does the camer need to be inside the oven? why not film trough the door mesh?

what other eletronics do you want to place inside the oven ?
and what parts of that need to be shielded?

what other stuff do you want to place inside the oven but not shielded ?

Wait, FunkyJive, you mean to say that when I was a little younger, it was a bad idea to stare into the microwave while it was running from a distance of 4 inches? Only reason I ever did it was cause I wanted to see what was going on.

As for an idea, film through a hole cut in the side of the inner box, with the camera where the circuits are, and a spare metal mesh covering the hole.

Onegaishimasu. I think it was the late Morihiro Saito Shihan that said once: "To question effectiveness is to kill effectiveness." A person is always better off just going to the dojo and seeing what kind of experiences you have.

In gassho,

Mark